There are many applications which involve the deposition of a substance onto a substrate. Commercially important methods include common printing techniques such as lithography, photocopying, spray depositing, and silk screening. These methods are quite adequate for depositing toners, dyes and other colorants which are readily fluidized, but are quite inadequate for depositing other materials such as metals, catalysts and so forth where the fluid form of the compound being deposited exists only under high temperature or other difficult working conditions.
Deposition of metals onto substrates is also known, but all known techniques are either imprecise, or are difficult to execute. For example, on the extremely simple end of the scale it is known to employ hot-dipping to apply zinc to nails, beams and other metal structures to retard oxidation of the substrate. It is also known to apply a metal containing paint to a canvas, glass or other substrate. Other simple procedures involve spraying and powder coating, but in all of these instances the techniques are generally imprecise, both in the area covered and in thickness of the coating applied. The coating also generally contains undesired impurities, and is very likely to have an uneven or roughened surface. There are more sophisticated techniques such as vapor deposition which can produce very precise layering of compounds on a substrate, but such techniques cannot produce complex patterns such as circuits.
Focusing then on techniques which are capable of depositing complex and precise patterns of difficult-to-fluidize compounds such as metals, the field can be divided into electro-plating and electroless plating methods. In electro-plating an entire surface of a substrate is coated with a metal, and portions of the coating are then selectively removed by etching or other approaches. Such processes are cumbersome at best, and have little or no advantage over electroless plating methods when producing complex and precise patterns such as those found in circuit boards or decorations, particularly where the substrate is non-conducting.
Electroless plating occurs through chemical rather than electrical reactions. Electroless plating can be divided into immersion systems and non-immersion systems. In immersion systems, the substrate to be coated is generally immersed in a bath of a metal or other coating solution. In the case of metal containing solutions, the metal is then precipitated out of solution and deposited on the substrate by the introduction of energy. Depending on the metal being deposited, the energy acts either to add electrons to the metal ions to form the metal. The energy for such processes is often in the form of a laser, with the wavelength selected to be absorbed by one or more of the molecules involved in the reaction, or with a dye. Unfortunately, there are several problems inherent in immersion processes. One very significant problem is the overall complexity. Immersion systems require some sort of immersion tank, with accompanying relatively large quantities of often toxic fluids. Another problem is that intricate and precise patterns are inherently difficult to achieve due to dispersion of energy within the immersion liquid. Still another problem is that good quality coatings are often unobtainable due to nonuniformity in concentrations and temperatures within the bath. Circulation pumps have been employed to minimize this problem, but such pumps add even greater complexity.
Non-immersion coating techniques overcome some of the problems associated with immersion methods of coating, but only at the cost of introducing yet other problems. In typical non-immersion coating systems, a non-conducting substrate such as glass, glass fibers, paper or plastic is entirely coated with a coating substance. In the case of printed circuit boards, for example, the coating substance which forms the electrically conducting paths is usually a highly conductive metal such as copper, silver or gold. Once coating is completed, the coated surface is masked in some way with a resist, and the masked surface is etched to form the final pattern. The various resists can be made in numerous ways, including photographic processes, silk screening processes and even hand painting.
All such processes known to the art are subtractive processes. Essentially the entire surface is coated with a substance, and then most of the substance is removed to leave a desired pattern. Such processes are inherently cumbersome, difficult and wasteful. Even with expensive equipment and experienced production staff, the quality of the finished product is difficult to control, and there is considerable waste in both the masking agent (resist) and the non-used coating.
So called additive methods of coating are known, but these methods are only partially additive--they are still partially subtractive in that a substantial portion (at least 5% by weight) of either the coating substance or a precursor to the coating substance must be removed to form the final pattern. U.S. Pat. No. 4,614,837 to Kane et al. (September, 1986), for example, describes a partially subtractive method in which a finely divided metal powder is placed on a substrate, and then compacted using a heated die. Under sufficient temperature and pressure, and particularly if the substrate is heat deformable, the method is said to produce usable electrically conductive pathways. Unfortunately, while the Kane et al. method manages to eliminate the need for masking, it raises additional problems. One disadvantage is that the process requires finely ground metal powders. Such powders are expensive, difficult to produce uniformly, and likely to be oxidized during storage. Further, dust and other impurities may also affect the quality of the metal powders, which may in turn cause problems during compaction. Dust collection systems may also be required to safeguard the health of the operatives. Still further, expensive dies having the pattern of the desired circuit also need to be manufactured. Finally, equipment to compress the metal powder onto the board has to be provided.
U.S. Pat. No. 5,576,074 to Weigel et al. (November, 1996) describes another partially subtractive method. In that patent a solution is provided which contains a salt or other metal containing compound, along with an amine developer. The solution is homogenized, and a uniform coating of the homogenized solution is placed on a surface of a substrate. Laser radiation is then trained on the solution according to a desired pattern which reduces the compound to a metal.
The Weigel process continues the prior art concept of indiscriminately coating a surface with a substance, and then using a laser to cause a metal to precipitate out of solution in a desired pattern. Such processes are impractical from several perspectives. The use of a laser energy, for example, results in a relatively slow rate of production because only a small spot can be exposed at any given time, and a certain amount of energy is required to energize the necessary redox reaction(s). Low power lasers will thus take a very long time to produce a pattern of much complexity, while high power lasers may damage the substrate, and may cause other quality problems. For this reason, processes such as the Weigel process may be adequate for preparing one of a kind prototypes, but are inherently impractical for mass production. Still further, lasers having the necessary power are relatively cumbersome and dangerous to use.
Moving beyond the problems inherent in using lasers to effect precipitation, there may also be difficulties in controlling the quality of the finished product because the redox reaction rate is likely to be dependent upon numerous factors, including thickness of the chemical mixture layer on the substrate, the rate of heat absorption into the substrate, and the energy required to excite the molecules of the chemical mixture. Still further, residual amounts of radiation absorbing dye and metallic toner are very likely to vitiate the quality of the finished product. Even if processes such as the Weigel process were perfected, it is thought that such processes are inherently incapable of resolving certain problems such as producing electrically conductive vias. Such processes are also incapable of being used in conjunction with ordinary printing techniques, such as stamping and ink jet type spraying.
U.S. Pat. No. 5,378,508 to Castro (January, 1993) discloses yet another example of partially subtractive deposition in which a surface is coated with a substance, a laser beam is trained on the substance to cause a metal or other product to adhere to the substrate, and then the excess coating substance is washed away to produce the desired pattern. Once again the same disadvantages apply, resulting primarily from the fact that coating material is first added to, and then partially removed from the substrate.
Thus, there is a need to provide coating techniques and apparatus which are capable of depositing a metal or other substance in a precise pattern on a substrate, without requiring substantial removal of either a precursor or the deposited substance from the substrate.